Ulcerative colitis is a chronic disease, characterized by intermittent periods of relapse and remission, that causes ulceration of the colon. Complications include colonic perforation, hemorrhage, pain, and extraintestinal manifestations of the disease including arthritis, skin lesions and liver disease. Treatment consists of 5-aminosalicylic (ASA) preparations (oral, topical or both) for mild to moderate disease and prednisone for more severe disease. Those patients who fail to respond to prednisone may require hospitalization, cyclosporin, methotrexate or colectomy. Preliminary data suggests the anticoagulant heparin may have efficacy in the treatment of active ulcerative colitis. Although the mechanism for heparin's effect in ulcerative colitis is unknown, it may relate to increasing the availability of blood and oxygen flow by the prevention of capillary microthrombi. Decreased rectal blood flow has been reported in ulcerative colitis. Several reports have described healing of perineal Crohn's disease involving the colorectum with hyperbaric oxygen therapy. This therapy has been shown to increase the dissolved oxygen content of plasma from 0.32 to 6.8 vol%. This increase in oxygen delivery may be important for the treatment of ischemic disease related to microvessel hypoperfusion. Recent data from both the acetic acid and trinitrobenzesulphonic (TNR) models of colitis in rats indicated disease severity could be limited with hyperbaric oxygen treatment. Specific Aims The primary objective of this pilot study is to determine if the potential exists for a clinical improvement in severe ulcerative colitis with hyperbaric oxygen treatment that could obviate or postpone the need for colectomy. The secondary endpoints will be to assess the effect of hyperbaric oxygen treatment on the symptoms of ulcerative colitis, endoscopic, hematologic and histologic parameters of inflammation, and patient quality of life. Subjects Five male or female patients (aged 18-64) will be recruited for this open-label pilot study. The duration of ulcerative colitis symptoms, as well as the duration of the current relapse will be recorded. Concurrent medications and their duration will be recorded. Treatment Each patient will be given 30 courses (2 hour duration) of hyperbaric oxygen treatment on a five day per week basis. This will consist of 100% oxygen at a pressure of 2.0 atmospheres absolute delivered in a multiplace hyperbaric chamber (Hyperbaric Oxygen Therapy Systems). Patients will continue whatever medical therapy they have been provided for their ulcerative colitis for the entire study duration. The only exception will be weaning of systemic corticosteroids.